The present invention relates generally to wireless communication devices, and in particular to a feed forward linear amplifier portion of a wireless communication device.
The frequency spectrum that is shared among radio communication devices is limited. Thus the ability of a transmitter to transmit as much information as possible in an allocated frequency spectrum or channel without interfering with other communication devices in adjacent channels is of great importance. To transmit as much information as possible in the allocated channel, digital communication systems typically modulate both the amplitude and phase of a radio frequency (RF) carrier. The amplitude modulation allows more information to be encoded on the carrier in a given channel than, if only the phase was modulated. However, the amplitude modulation puts additional requirements on the transmitter that would not exist if only the phase of the RF carrier was modulated.
These additional requirements are due to the inherent nonlinear effects resulting from the amplification of an amplitude-modulated signal by an RF power amplifier. Due to the nonlinear characteristics of the RF power amplifier, signal distortion that typically includes high order non-linearities (e.g., third order, fifth order, seventh order, and ninth order intermodulation products) is introduced to an amplified output signal. When the output signal is transmitted, the distortion creates undesirable interference, for example at frequencies corresponding to the third order, fifth, seventh, and ninth order intermodulation distortion (IM) products. A number of prior art signal processing techniques have been developed to compensate for the nonlinear characteristics of RF power amplifiers. One such technique involves the use of a carrier cancellation circuit and an IM reduction circuit in a feed forward amplifier. In general, feed forward amplifiers separate out distortion components generated by the RF power amplifier and reference signal modulation products introduced to an input signal to create an error signal. The error signal is then amplified and added to the RF power amplifier""s output with an amplitude, phase, and delay adjustment for cancellation of distortion components and reduction of IM products.
FIG. 1 is a block diagram of an exemplary feed forward amplifier 100 of the prior art. Feed forward amplifier 100 includes a main signal path 102 having two low frequency modulators 104, 110 and an RF power amplifier 116. An input signal 101 having a carrier component is sourced to main signal path 102, where the signal is routed to an amplitude modulator 104 and a phase modulator 110 that respectively modulate an amplitude and a phase of the input signal. Amplitude modulator 104 includes an amplitude reference signal generator 106 coupled to an amplitude adjuster 108. Amplitude reference signal generator 106 produces a first low frequency pilot tone 107, such as a 13 KHz tone, that is conveyed to amplitude adjuster 108. Amplitude adjuster 108 then varies an amplitude of input signal 101 in response to pilot tone 107 to impose a reference amplitude modulation upon input signal 101. Phase modulator 110 includes a phase reference signal generator 112 coupled to a phase adjuster 114. Phase reference signal generator 112 produces a second low frequency pilot tone 113, such as an 11 KHz tone, that is conveyed to a phase adjuster 114. Phase adjuster 114 then varies a phase of input signal 101 in response to pilot tone 113 to impose a reference phase modulation upon input signal 101. Amplitude adjuster 108 and phase adjuster 114 respectively also vary an amplitude and a phase of input signal 101 in response to respective control signals 156, 158 received from a carrier cancellation controller 154.
The amplitude and phase modulated input signal is applied to an RF power amplifier 116, which amplifies the modulated input signal to produce an amplified signal 118. Amplified signal 118 includes a carrier component, distortion components introduced to the amplified signal due to the nonlinearities of amplifier 116, and reference modulation components resulting from the introduction of pilot tones 107 and 113 to input signal 101 by amplitude and phase adjusters 108 and 114. Amplified signal 118 is sampled by a first output signal coupler 120 to produce a first sampled amplified signal 121. Sampled amplified signal 121 is then supplied as one input to a signal combiner 132. Also routed to combiner 132 is a time-delayed, sampled version 131 of input signal 101, which is routed to the combiner via a delay circuit 130. Signal combiner 132 subtracts sampled input signal 131 from sampled amplified signal 121 to provide carrier cancellation and to produce an error signal 133 comprising the distortion and reference modulation components contained in sampled amplified signal 121.
Error signal 133 is then supplied to a amplitude adjuster 134 via a signal coupler 150. Amplitude adjuster 134 varies an amplitude of error signal 133, and thereby of the modulation component contained in the error signal, in response to an amplitude control signal 137 provided by an amplitude reference signal receiver 136. The amplitude adjusted error signal is then applied to a phase adjuster 138, which varies a phase of the amplitude adjusted error signal, and thereby of the modulation component contained in the amplitude adjusted error signal, in response to a phase control signal 141 provided by a phase reference signal receiver 140. The amplitude and phase adjusted error signal is then applied to an error amplifier 142. Error amplifier 142 amplifies the amplitude and phase adjusted error signal and provides the amplified amplitude and phase adjusted error signal to a second output signal coupler 124.
Second output signal coupler 124 also receives a time-delayed version of amplified signal 118 from RF power device 116 via a delay circuit 122. Signal coupler 124 combines the amplified amplitude and phase adjusted error signal with the time-delayed amplified signal to effectively subtract the amplified amplitude and phase adjusted error signal from the time-delayed amplified signal and to produce a corrected output signal 128. Corrected output signal 128 is then sampled by a third output signal coupler 126 to produce a sampled corrected output signal 127. Sampled corrected output signal 127 is conveyed to a first amplitude detector 144 that detects an amplitude of any residual modulation component remaining in the sampled corrected output signal.
Since both amplitude and phase modulation components may be present in sampled corrected output signal 127, amplitude detector 144 provides the detected residual modulation to each of phase reference signal receiver 140 and amplitude reference signal receiver 136 via a band pass filter 146. Band pass filter 146 is designed to filter out any non-residual reference modulation component of the signal conveyed by detector 144. Based on an amount of detected residual modulation, amplitude reference signal receiver 136 and phase reference signal receiver 140 then each adjust their respective control signals 137 and 141 in order to minimize the amount of detected residual reference modulation. By adjusting control signals 137 and 141 such that the amount of detected residual reference modulation is minimized, reference signal receivers 136 and 140 minimize the amount of IM included in corrected output signal 128.
The carrier cancellation performed by signal combiner 132 is not perfect, and as a result error signal 133 further includes a residual carrier component. Signal coupler 150 samples error signal 133 to produce a sampled error signal that the signal coupler provides to a second amplitude detector 152. Amplitude detector 152 detects an amplitude of the sampled error signal and provides the detected amplitude to carrier cancellation controller 154. Based on the detected amplitude, controller 154 produces multiple control signals 156, 158 that are intended to minimize the amplitude of the sampled error signal detected by amplitude detector 152, thereby optimizing the carrier cancellation performed by signal combiner 132. Controller 154 conveys a first control signal 156 of the multiple control signals to amplitude adjuster 108, in response to which the amplitude adjuster adjusts an amplitude of input signal 101 so as to minimize the detected amplitude of error signal 133. Controller 154 conveys a second control signal 158 of the multiple control signals to phase adjuster 114, in response to which the phase adjuster adjusts an phase of input signal 101 so as to further minimize the detected amplitude of error signal 133.
The modulation of input signal 101 by modulators 104, 110 in order to apply one or more low frequency pilot tones 107, 113 to the signal and simultaneously to reduce a detected amplitude of error signal 133 imposes tuning constraints upon amplifier 100. For optimal IM performance of amplifier 100, pilot tones 107, 113 should be set at a power level designed to produce sidebands on the input signal carrier that are typically 30 dB or more below the carrier. However, the application of pilot tones 107, 113 to input signal 101 imposes carrier cancellation tuning constraints upon feed forward amplifier 100, resulting in sub-optimal carrier cancellation. Furthermore, the application of pilot tones 107, 113 to input signal 101 can result in a larger than necessary average and peak of error signal 133, with a peak-to-average ratio as large as 20 dB, requiring a higher power, more costly, error amplifier 142 in order to avoid clipping by the error amplifier and a resulting introduction of undesired distortion into the amplified error signal produced by the error amplifier.
Therefore a need exists for a method and apparatus for tuning a feed forward amplifier that permits a minimization of intermodulation distortion components in an output signal produced by the amplifier while further permitting an optimization of the carrier cancellation performance of the amplifier.